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Appendix VI.                          1067

may be obtained regarding compound stress in such pieces, and
thus be a means of correcting or verifying the theories on which
we now rely.

The bed B B supports the straining cylinder A, the plunger of
which is connected by the head D to a sliding trough E E that
carries the straining head z. This head is run on rollers to any re-
quired position, and is then fixed by large square bolts that are shot
out by right and left-handed screws into the square holes T T. The
weight of the sliding trough is also taken by rollers as at G, of
which there are several, resting on rails F F ; and the whole, thus
far, constitutes the straining system. The weighing apparatus
consists of a steelyard v and counterbalance w, connected to a
bell-crank lever p, which bears against a crosshead N, the total
leverage being 600: i; and the weighing load is further trans-
mitted by the tension rods R R and crossheads M s, to the straining
heads K and L, all supported on rollers. As the head z always
moves rightward under the hydraulic pressure in cylinder A, and
the heads K L tend to move leftward by virtue of the steelyard
load, the positions for specimens in shear, tension, or compression
will be as indicated in the figure. It should be mentioned, how-
ever, that the head L is also used for beam deflections, when a
special support for the beam is placed horizontally across the
bed c, and the head z is brought to a convenient position for the
experiment. The whole machine is very long, some 130 ft., so it
has been found convenient, in the diagram, to break the picture
at four places, for the actual figure would otherwise have been
three or four times the length of what is here shewn. The
trough being very heavy, is moved backward and forward to
starting position, by a piston contained in the smaller cylinder H,
and connected by a rod to the lug j on the trough. This method
economises the high-pressure water, which at lyoolbs. pressure in
cylinder A of 26 ins. diameter would otherwise be a great loss.
The lever P and crosshead N are slung from the bracket x; and
the fulcra at Q are 2 ins. apart, so axe easily verified. The jockey
weight w acts as counterpoise at first, but being moved leftward
to gradually balance increasing load, represents 60 tons on the
specimen when in extreme position. It is then exchanged for
one of the weights at u, which is lifted and put in place by the